1. Field of the Invention
This invention relates to a method of manufacturing a heat-resistant plastic vessel through processes of making a preform, a primary blow-mold object and a secondary blow-mold object, and to a primary blow-mold object made by this method.
2. Description of the Related Art
One method of fabricating a heat-resistant plastic vessel is known in U. S. Pat. No. 4,836,971. With this method, firstly an injection-molded preform is subject to the biaxial stretch blow molding process to obtain a primary blow-mold object, which is vertically longer than a heat-resistant hollow plastic object as a final product. Then, this primary blow-mold object is heat-shrunk, being subject to the biaxial stretch blow molding process to obtain the heat-resistant hollow object as the final product.
The secondary blow-mold object, i.e. a heat-resistant vessel such as a bottle, has to meet a variety of requirements. One of them is that the heat-resistant hollow object should have a buckling strength enough to prevent its deformation and breakage caused by an axial compressive force.
To increase the buckling strength, the hollow object usually has a ring-shaped concave or convex reinforcing rib on its body. Such reinforcing rib can be easily formed by making a concave or convex portion on the inner surface of secondary blow cavity molds.
According to studies of researchers including the present inventors, it has been confirmed that a large thickness portion around the body of the hollow object is very effective to assure the buckling strength. Such large thickness portion may be positioned either at the reinforcing rib or at any position on the body regardless of the reinforcing rib.
It is however very difficult to form a large thickness portion around the body of the primary blow-mold object. It is theoretically feasible to heat the primary blow-mold object by applying different temperatures thereto so that the portions of the object may be differently heated, and that the hot portion is stretched to be thinner than the cold portion which is less stretched. The present inventors performed experiments, finding that the large thickness portion is formed according to the foregoing theory, but that it is much more difficult to control a region to be thickened to obtain a large thickness portion in the final product. In other words, a very large region was thickened even if temperatures applied to heat the primary blow-mold object were controlled very carefully. Specifically, the large thickness portion is formed not only at the target area but also at areas around the target area, or the desired thickness could not be attained. In such a case, more resin material will be required to make a preform, which will become undesirably heavy.